What is ‘COVID Arm’? Researchers are finally beginning to understand this side effect of vaccines

If you had received a COVID-19 vaccine and a few days later had a swollen red rash at the injection site, you might have gotten a ‘COVID arm’. This annoying (but ultimately harmless) side effect of the coronavirus vaccine is something that researchers are now beginning to understand a little better.

The symptoms of what is known as COVID arm are redness, swelling and tenderness at the injection site that develop eight or more days after the vaccination, according to a new report in the New England Journal of Medicine. Examination of clinical trial data for the Modern mRNA vaccine found that the response usually disappeared within four or five days.

To put this in perspective, the researchers note that approximately 84% of the people in the trials had a reaction, such as pain, shortly after the shot at the injection site. But only 0.8% of people (244 out of about 30,000) experience these delayed skin reactions after their first dose. However, the researchers note that the trial data do not give a complete picture of what the reactions may include and that they do not distinguish between reactions after the first and second dose of the vaccine.

The researchers therefore examined 12 case reports of people who developed delayed skin reactions after receiving the Moderna vaccine. Most people noticed that their symptoms started on day eight or nine after receiving the first dose of the vaccine, but the reaction of one person appeared on day four and one developed it on day 11. Most patients have itching, redness, swelling and pain. Interestingly, not everyone who developed this reaction after the first dose also received one after the other: Of the twelve patients in this study, only half reported having a similar reaction after the second dose ( three of those have the second time).

Although researchers still do not know exactly what causes this reaction, this pattern of symptoms and a skin biopsy of another patient (who was not one of the other 12 in the study) give them some clues. The biopsy suggests that the body’s T cells, a type of immune cell that can limit the effects of an invasive virus, may be behind these delayed hypersensitivity reactions.

Perhaps the biggest takeaway from these results is that the fact that you have one of these delayed reactions to the first dose of COVID-19 vaccine does not mean that you cannot get the second. “We can now give the assurance that it is safe to get the second #modernavaccin, even if you had a major local reaction to the first skin on the first shot,” said Esther E. Freeman, MD, Ph.D. ., director of global health dermatology at Massachusetts General Hospital, associate professor of dermatology at Harvard Medical School, and one of the authors of the study, wrote on Twitter.